tidy: Add a rule ensuring that // comments are followed by a space in Rust (#38698)

This shows up sometimes in code reviews, so it makes sense that tidy
enforces it. `rustfmt` supports this via comment normalization, but it
does many other things and is still an unstable feature (with bugs).

Testing: There are new tidy tests for this change.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

python/tidy/test.py

@@ -117,8 +117,11 @@ class CheckTidiness(unittest.TestCase):
 
     def test_rust(self):
         errors = tidy.collect_errors_for_files(iterFile("rust_tidy.rs"), [], [tidy.check_rust], print_text=False)
+        self.assertEqual("Comments starting with `//` should also include a space", next(errors)[2])
         self.assertEqual("use &T instead of &Root<T>", next(errors)[2])
         self.assertEqual("use &T instead of &DomRoot<T>", next(errors)[2])
+        self.assertEqual("Comments starting with `//` should also include a space", next(errors)[2])
+        self.assertEqual("Comments starting with `//` should also include a space", next(errors)[2])
         self.assertNoMoreErrors(errors)
 
         feature_errors = tidy.collect_errors_for_files(iterFile("lib.rs"), [], [tidy.check_rust], print_text=False)

python/tidy/tests/rust_tidy.rs

@@ -2,6 +2,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+//! This is a module, but this comment shouldn't trigger an error.
+//* This is also a module doc comment *//
+//This comment should trigger an error, because of missing space.
+
 use    app_units::Au;
 use raqote::{GradientStop, Source, SolidSource};
 use raqote::{Source, SolidSource},
@@ -78,4 +82,11 @@ impl test {
     } else {
         let xif = 42 in {  xif  } // Should not trigger
     }
+
+    //Comment with no space following it, should trigger an error.
+    let a = 3;//This should cause a problem.
+    //~^ This is a compile-test comment and shouldn't cause a problem.
+    // This comment has a URL, but it shouldn't trigger an error http://servo.org.
+    // This comment has a URL, but it shouldn't trigger an error http://servo.org.
+    /* This is another style of comment, but shouldn't trigger an error. */
 }

python/tidy/tidy.py

@@ -555,6 +555,9 @@ def check_rust(file_name: str, lines: list[bytes]) -> Iterator[tuple[int, str]]:
         return
 
     for idx, line in enumerate(map(lambda line: line.decode("utf-8"), lines)):
+        for match in re.finditer(r"(;|\s|^)//\w", line):
+            yield (idx + 1, "Comments starting with `//` should also include a space")
+
         line = re.sub(r"//.*?$|/\*.*?$|^\*.*?$", "//", line)
         rules = [
             # There should be any use of banned types: